Monthe was the Vice Chairman of the Advisory Committee on Administrative and Budgetary Questions (ACABQ) before taking office at the United Nations. His other diplomatic postings included Special Adviser to the President of the fifty-ninth session of the General Assembly; Inspector General in Cameroon’s Ministry of External Relations; Chair of the Anti-Corruption Unit in the Ministry; Ministry Secretary-General; Chargé de Mission in the Diplomatic Division of the General Secretariat at the Presidency; Technical Adviser in the Ministry of External Relations; and responsibity for consular affairs in the Protocol Department.

From 1977 to 1989, he served a prior term in New York. This term included postings as Second Secretary, First Secretary, Second Counselor and First Counselor for Cameroon's Permanent Mission to the United Nations.